1. Field of the Invention
The present invention relates to a uni-morph, bi-morph or other type of piezoelectric or electrostrictive film element which generates or detect displacement or force in the form of bending, deflection or flexure, and which can be used for actuators, filters, display devices, transformers, microphones, sounding bodies (such as loudspeakers), various resonators, or vibrators, sensors, and other components or devices. The term "element" used herein is an element capable of transducing or converting an electric energy into a mechanical energy, i.e., mechanical force, displacement, strain or vibrations, or transducing such a mechanical energy into electric energy.
2. Discussion of Related Art
In recent years, in the fields of optics and precision positioning or machining operations, for example, there have been widely used and increasingly demanded an element whose displacement is controlled for adjusting or controlling an optical path length or the position of a member or component of a device, on the order of fractions of a micron (.mu.m), and a detecting element adapted to detect infinitesimal displacement of a subject as an electric change. To meet this need, there have been developed piezoelectric or electrostrictive film elements used for actuators or sensors, which elements comprise a piezoelectric material such as a ferroelectric material, and utilize the reverse or converse piezoelectric effect to produce a mechanical displacement upon application of an electric field to the piezoelectric material, or utilize the piezoelectric effect so as to produce an electric field upon application of a pressure or mechanical stress. Among these elements, a conventional uni-morph type piezoelectric/electrostrictive film element has been favorably used for a loudspeaker, for example.
There have been proposed ceramic piezoelectric/electrostrictive film elements used for various purposes, as disclosed in JP-A-3-128681 and JP-A-5-49270 filed by the assignee of the present application. An example of the disclosed elements has a ceramic substrate which has at least one window, and is formed integrally with a thin diaphragm which closes the window or windows so as to provide at least one thin-walled diaphragm portion. On an outer surface of each diaphragm portion of the ceramic substrate, there is formed a piezoelectric/electrostrictive unit (hereinafter referred to as P/E unit) which is an integral laminar structure consisting of a lower electrode, a piezoelectric/electrostrictive layer (hereinafter referred to as a P/E layer) and an upper electrode. This P/E unit is formed by a suitable film-forming method on the relevant diaphragm portion of ceramic substrate. The thus formed piezoelectric/electrostrictive film element is relatively small-sized and inexpensive, and can be used as an electromechanical transducer having high reliability. Further, this element has a quick operating response, and provides a relatively large amount of displacement by application of a low voltage, with a relatively large magnitude of force generated. Thus, the above-described element is advantageously used as a member for an actuator, filter, display device, sensor or other component or device.
To produce the piezoelectric/electrostrictive film element as described above, the lower electrode, P/E layer and upper electrode of each P/E unit are laminated in this order on the diaphragm portion of the ceramic substrate by a suitable film-forming method, and is subjected to heat treatment (firing) as needed, so that the P/E unit is formed integrally on the diaphragm portion. A further study of the inventors of the present invention revealed that the piezoelectric/electrostrictive characteristics of the piezoelectric/electrostrictive film element are deteriorated due to the heat treatment (firing) effected during the formation of the P/E unit, more specifically, the P/E layer.
That is, the P/E layer suffers from stresses due to firing shrinkage of the P/E layer or P/E unit which is in contact with the diaphragm portion of the ceramic substrate, during the heat treatment of the P/E layer. As a result, the P/E layer may not be sufficiently sintered due to the stresses, and still suffers from stresses remaining therein after the firing. In this case, the piezoelectric/electrostrictive film element cannot exhibit its inherent piezoelectric/electrostrictive characteristics.
In order to increase the sinterability and density of the P/E layer so as to improve the piezoelectric/electrostrictive characteristics of the film element, the firing temperature of the P/E layer may be increased, or the thickness of the diaphragm portion which bears the P/E layer thereon can be reduced. Yet these solutions are not sufficient to improve the density of the P/E layer, and the stresses remaining after firing of the P/E layer may deteriorate the piezoelectric/electrostrictive characteristics. In particular, such residual stresses may reduce the amount of displacement of the diaphragm portion upon actuation of the P/E unit. Further, the reduction in the thickness of the diaphragm portion makes it more difficult to produce the ceramic substrate.
The above-described conventional solutions may also cause a great amount of deflection of the diaphragm portion, which results in a reduced amount of displacement of the diaphragm portion upon actuation of the P/E unit. When two or more adjacent P/E units are actuated at the same time, in particular, the amount of displacement of these units are considerably reduced, as compared with the case where a single P/E unit is actuated. Due to the great amount of deflection of the diaphragm portion, the piezoelectric/electrostrictive film element may break during manufacture or use of the element, resulting in reduced operating reliability.